A context reference for the present invention is Pressure-Balanced Rod Piston Control System for a Subsurface Safety Valve U.S. Pat. No. 6,173,785 B1. The operating mechanism for the isolation valve for subsea use will be very similar to that which is shown in FIGS. 1 and 2 of this patent. In fact, the pressure balance functionality of the operating mechanism unchanged; however, with the present invention it will be improved to offer a local visual, tactile indication of the valve's position to a diver or Remotely Operated Vehicle (ROV).
Many subsea isolation valve installations such as those involving a semisubmersible rig use hydraulically operated plug or ball valves coupled to large hydraulic operators for quarter turn operation between open and closed positions. Because of the high operating pressures and the line sizes involved these valves are extremely heavy and have a fairly large profile exposed to wave action. As a result such weight must be offset with buoyancy and the fixation system for the rig has to resist the loads from wave action and underwater currents against the large profiles of these valves and their operators.
To address these issues the present invention proposes to use subsurface safety valves (SSV) of the type well known in the art in place of the heavy and high profile valves now being used as subsea isolation valves. Apart from the above reference, the following references illustrate the state of the SSV art and position indication art: U.S. Pat. No. 8,176,975; US 20060157240; U.S. Pat. Nos. 7,533,693; 3,077,179; 5,890,450 and US 20120234410.
Apart from using an SSV as a subsea isolation valve, other novel features of the present invention are the ability to put an external position indication to that valve and the manner in which such an external indication system operates. The internal workings of the known SSV need not be modified. Instead, the hydraulic system operates the local indication feature. The hydraulic system normally has discrete lines leading to opposed ends of an operating piston for the SSV. Movement of the operating piston moves a flow tube against a flapper for the open position. A closure spring closes the valve by pushing up the flow tube when pressure is released in the operating control line. The force closed line goes to the back side of the operating piston and can be used to force the operating piston back up to close the valve when the closure spring fails to do so. The present invention provides local hydraulic taps into those two control lines for local operation by a diver or an ROV. The local position indicator is preferably integrated into the force close line for the main reason that such line operates at significantly lower pressures than the pressures seen in the operating control line. The position indicator is a volumetric displacement device. The volume displaced by the operating piston is the volume pushed to the position indicator. Depending on the relative diameters of the operating piston compared to the indicating piston the movement of the indicating piston will be the same or different than the movement of the operating piston. In this manner the application of the known SSV design to a subsea application will not require internal valve modifications to meet the requirements of API-6A/6DSS/17D standards. The indicator mechanism can be configured to be depth insensitive to seawater. The indicator can be calibrated as part of the manufacturing and assembly process in conjunction with its associated valve to compensate for any trapped compressible fluids in the system. The indicator piston can displace trapped fluid against a floating piston to isolate the indicating piston and its seals from seawater. These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention can be determined from the appended claims.